Si(OR)m Xn Yp wherein R is a hydrocarbyl group, X and Y which may be the same or different are a hydrocarbyl group, halogen atom or a carbonyloxy group represented by the formula, ##STR1## wherein R' is a hydrocarbyl group, m is 1???4, n is 0???3, p is 0???3 and m+n+p is 4.